This disclosure pertains, inter alia, to X-ray projection-exposure (xe2x80x9cmicrolithographyxe2x80x9d) apparatus for transferring, for instance, circuit patterns defined on a photomask (either a mask or a reticle) onto a substrate such as a semiconductor wafer. The X-ray projection-exposure apparatus employ an X-ray beam having a wavelength in the range of 1 to 30 nm that transfers the pattern by passing through a reflection-type imaging-optical system. This disclosure also pertains to exposure methods performed using such an X-ray projection-exposure apparatus, and to semiconductor devices manufactured using the X-ray projection-exposure apparatus.
Exposure apparatus used for manufacturing semiconductor devices are typically configured to project, and thus xe2x80x9ctransfer,xe2x80x9d a circuit pattern, defined on a mask or reticle (termed a xe2x80x9cmaskxe2x80x9d herein), via a projection-optical system onto a suitable substrate such as a semiconductor wafer. During projection-exposure the mask is situated at an object plane and the substrate is situated at an image plane. In most projection-exposure apparatus currently in use (that utilize light in the deep-UV range), both the mask and the optical elements of the projection-optical system are transmissive to the light used for performing pattern transfer, and hence are termed xe2x80x9ctransmissivexe2x80x9d optical systems. For example, a projection-exposure apparatus still currently in wide use utilizes, as an exposure-light source, xe2x80x9ci-linexe2x80x9d light produced by a high-pressure mercury lamp. Other deep-UV projection-exposure apparatus utilize, for example, a KrF excimer laser as a source of exposure light.
A conventional transmissive optical system 40 is depicted conceptually in FIG. 5. The depicted apparatus includes a light source 49, an illumination-optical system 50, a projection-optical system 51, a mask stage 53 for holding a mask 52, a wafer stage 55 for holding a wafer 54, a detection system 56 for detecting alignment marks on the wafer 54, and a detection system 58 for detecting alignment marks on the mask 52. A light beam, used for pattern transfer, downstream of the mask 52 is denoted as item 57a, and downstream of the projection-optical system 51 is denoted as item 57b. In FIG. 5 the beam 57a, 57b is shown propagating along the optical axis of the projection-optical system 51.
The apparatus 40 of FIG. 5 also includes an optically based system 59a, 59b for detecting the surface position of the wafer 54. The surface-position-detection system includes a light source 59a that obliquely illuminates a light beam onto the surface of the wafer 54. Light of the beam reflected from the wafer 54 is detected by a photodetector 59b. Thus, the position of the wafer surface, in a direction along the optical axis, is detected. Examples of surface-position-detection systems of this type are described in Japan laid-open (Kxc3x4kai) Patent Application No. Hei 6-283403, Japan Kxc3x4kai Patent Application No. Hei 8-64506, and Japan Kxc3x4kai Patent Application No. Hei 10-214783. Such a detection system also may be used for detecting positions of a mark formed on the wafer and/or positions of a mark formed on the mask 52.
The pattern defined on the mask 52 can be configured for projection, by the projection-optical system 51, onto the wafer 54 at unity magnification or with demagnification. Demagnification is characterized by the image on the wafer being smaller, by a demagnification factor established by the projection-optical system 51, than the corresponding pattern on the mask 52.
The projection-optical system 51 normally comprises multiple lenses or the like that collectively function to form an image of the mask pattern on the surface of the wafer 54. The entire mask pattern can be exposed in one exposure xe2x80x9cshot,xe2x80x9d or may require multiple exposure shots, depending upon the optical field (exposure-image field) of the projection-optical system 51 relative to the size of the pattern as projected onto the wafer. For example, if the projection-optical system 51 has an optical field of 20-mm square, then a die (or multiple dies) having a total area of no greater than 20-mm square can be exposed on one shot.
On the surface of the wafer 54, patterns for microcircuits are projected and formed layer-by-layer in a superposed manner. Exposure and formation of the requisite number of layers results in formation of a micro-electronic device in which the layers are interconnected with each other in a three-dimensional manner. These layers must be formed in a manner requiring extremely accurate registration (xe2x80x9coverlayxe2x80x9d) of each new layer with existing layers formed in previous exposures. To achieve high overlay accuracy, the apparatus 40 of FIG. 5 typically also includes respective devices for detecting the positions of the mask 52 and wafer 54 as exposures are being made. Each device normally includes a respective interferometer and the respective mark-position-detection system 58, 56. As the interferometers measure the respective positions of the mask stage 53 and wafer stage 55 in real time, the mark-position-detection systems 58, 56 optically detect respective alignment marks defined on the wafer 54 and the mask 52.
For example, the mark-position-detection system 56 can be configured as an optical microscope that produces a magnified image of the detected mark on the wafer 54. The system 56 includes an image detector, such as a charge-coupled device (CCD), for detecting the magnified image. In many conventional apparatus, the mark-position-detection system 56 is mounted laterally adjacent the projection-optical system 51 due to space constraints. An example of such a mark-position-detection system is disclosed in Japan Kxc3x4kai Patent Application No. Hei 5-21314.
FIG. 6 depicts a representative relationship, at the image plane, between the optical field (exposure-image field) of the projection-optical system 51 and xe2x80x9cdetection centersxe2x80x9d associated with the mark-position-detection system 56. The hatched area in the center of the figure corresponds to the exposure-image field 62, which has a center 61. In this example, the exposure-image field 62 is rectangular. A straight line 64 denoted by a dot-dash line extends laterally from the center 61 in the X-direction. The wafer stage 55 is configured to move the wafer 54 in directions parallel to the line 64. Another straight line 65, denoted by a dot-dash line, extends vertically in the Y-direction (at a right angle to the line 64) from the point 61.
In a conventional projection-exposure apparatus, the central axis (i.e., the optical axis) of the projection-optical system 51 typically passes (in a Z-direction) through the center 61 of the exposure-image field 62. The reason for this configuration is that the optical elements (lenses and/or mirrors) of the projection-optical system 51 typically are axially symmetrical in shape and situated along the optical axis, and the exposure light passing through the projection-optical system 51 is kept at or close to the optical axis to minimize optical aberrations. As a result, the exposure-image field on the image plane typically is located near the optical axis.
In instances in which the mark-position-detection system 56 (FIG. 5) is optical in configuration, the detection center 63 usually is situated at a position that is separated from the center point 61 of the exposure-image field 62. The distance between these points 61, 63 is a defined distance denoted by xe2x80x9cBLxe2x80x9d in the figure. Establishing the distance BL prevents the mark-position-detection system 56 from interfering with the projection-optical system 51. In this regard, the detection center 63 denotes the intersection of the optical axis of the mark-position-detection system 56 with the image (wafer) plane. In this instance, the distance BL is essentially equal to a dimension that is the sum of the radius of the optical column of the projection-optical system 51 and the radius of the optical column of the mark-position-detection system 56. Assuming that the wafer stage 55 is configured to move along an X-direction drive axis that is parallel to the line 64 and to move along a Y-direction drive axis that is parallel to the line 65, the detection center 63 can be situated at any of four positions 63a, 63b, 63c, 63d. The positions 63a, 63care located on the line 64 and separated by respective distances BL from the center 61 of the projection-optical system 51; similarly, the positions 63b, 63d are located on the line 65 and separated by respective distances BL from the center 61 of the projection-optical system 51.
Whenever an exposure operation is being conducted, the respective distances BL (termed xe2x80x9cbase linesxe2x80x9d) between the center 61 of the exposure-image field and any of the detection centers 63a, 63b, 63c, 63d are measured in advance. As the position of a mark on the wafer 54 is being measured by the mark-position-detection system 56, the coordinates of a position on a wafer at which exposure is to be performed are determined from the base line BL and from the relationship of a previously determined mark position and the position on the wafer at which an exposure is to be made. The wafer stage 55 is moved as required to place the coordinates, at which exposure is to be made, coincident with the center 61 of the exposure-image field 62. Thus, the exposure-image field 62 is positioned at the desired position on the wafer 54 to achieve projection of an image at the desired location on the wafer surface.
Recently, with relentless demand for increasingly higher integration, smaller miniaturization, and greater performance of microelectronic circuits, correspondingly greater resolution has been demanded of projection-exposure apparatus. In general, the resolving power W of a projection-exposure apparatus is a function of exposure wavelength xcex and the numerical aperture NA of the projection-optical (xe2x80x9cimaging-opticalxe2x80x9d) system. The relationship is expressed as follows:
W=k1xcex/NA
where k1 is a constant. According to this expression, improved resolution can be achieved by decreasing the wavelength of the exposure light and/or by increasing the numerical aperture. By way of example, in conventional projection-exposure apparatus that utilize i-line light (xcex=365 nm), a resolution of 0.5 xcexcm can be obtained with an imaging-optical system having an NA of approximately 0.5. Since it is difficult to fabricate imaging-optical systems having NA greater than 0.5, the primary thrust in the effort to achieve better resolution has been toward shortening the wavelength of the exposure light. In this regard, exposure apparatus have been developed that utilize excimer-laser light. For example, a KrF excimer laser produces deep-UV light of wavelength 248 nm, and an ArF excimer laser produces deep-UV light of wavelength 193 nm. Based on these data, a resolution of 0.25 xcexcm or less is expected from the use of KrF excimer laser light for exposure, and a resolution of 0.18 xcexcm or less is expected from use of ArF excimer laser light for exposure. Even shorter wavelengths have been investigated. For example, X-ray light having a wavelength of 13 nm has been used as exposure light, yielding a resolution of 0.1 xcexcm or less.
With respect to X-ray projection-exposure apparatus, no known lens materials exhibit adequate transmissivity to the X-ray light. Consequently, all the optical elements in imaging-optical systems in such apparatus must be reflective (mirrors) rather than refractive (lenses). Unfortunately, it is very difficult to design a reflective optical system having a wide on-axis optical field that produces imaging of acceptable quality for microlithography. As a result, multiple exposures of respective portions of a pattern are made to complete exposure of a complete pattern for a layer of the device. For consistent imaging quality, the exposure-image field can be formed off-axis (such a system is termed an xe2x80x9coff-axisxe2x80x9d optical system), wherein the image is formed in, e.g., a ring-shaped or annular field in which very high image resolution can be obtained due to excellent correction of aberrations. Using such an optical system, an image of a complete pattern for a circuit layer can be formed on the wafer by scanning the mask and wafer. Scanning exposure enables, for example, exposure of a die having dimensions of 20-mm square or larger using an imaging-optical system of which the exposure-image field is substantially smaller. Using this approach, acceptable X-ray imaging over a wide area can be achieved.
FIG. 7 is a conceptual view of a conventional X-ray projection-exposure apparatus 70. The major components of the apparatus 70 are a source 77 of an illumination beam 79a, an illumination-optical system 78 that receives the beam 79a and directs a corresponding X-ray beam 79b toward a mask 72, a mask stage 73, a projection-optical system 71 that receives the beam 79c reflected from the mask 72 and directs a corresponding imaging beam 79d toward a wafer 74, and a wafer stage 75.
The source 77 can be any suitable source of X-ray light, such as a discharge-plasma X-ray source. The illumination-optical system 78 comprises multiple reflective optical elements (mirrors) and filters as required to form the illumination beam 79b as a xe2x80x9chollow beamxe2x80x9d having an annular transverse section, thereby configuring the beam for a ring-shaped illumination field on the mask 72.
The projection-optical system 71 comprises multiple reflective mirrors and the like as required to form an image of the portion of the mask 72 illuminated by the illumination beam 79b. So as to have high reflectivity to incident X-ray light, the mask 72 and mirrors of the projection-optical system 71 each have a multilayer-film coating on their respective reflective surfaces. The projection-optical system 71 retains the general shape of the illumination field and forms a corresponding image, via the beam 79d, on the surface of the wafer 74. I.e., the projection-optical system 71 has a ring-shaped exposure-imaging field that transfers to the wafer 74 the respective pattern portion defined in the corresponding ring-shaped illuminated area of the mask 72.
As noted above, the mask 72 is reflective and defines its respective circuit pattern on a multilayer-film reflective surface of the mask. Since the mask 72 is reflective, the projection-optical system 71 is non-telecentric on the mask side.
Since X-rays of wavelength ranging from 1 to 30 nm are greatly attenuated by the atmosphere, at least the propagation paths of the X-ray beam in the system of FIG. 7 must be maintained, during actual operation, in a subatmospheric-pressure (more specifically, high vacuum) environment or, alternatively, a helium environment. High vacuum is the more common and more preferred. As a result, the portion of the apparatus between the source 77 and the wafer 74 typically is contained in a vacuum chamber (not shown).
As noted above, micro-electronic devices typically are formed as multiple patterned layers formed superposedly relative to each other so as to interconnect three-dimensional dimensionally with each other. As a result, during microlithography performed using an X-ray projection-exposure apparatus 70, each subsequently applied layer is formed so as to overlay and register with the previously formed layer. To perform this overlay-exposure with high accuracy, it is necessary to include with the apparatus 70 devices capable of accurately and precisely detecting the position of the mask 72 and wafer 74 relative to each other during exposure. A typical device includes an interferometer and a mark-position-detecting system (not shown). The respective interferometers determine the position of the mask stage 73 and the position of the wafer stage 75 in real time. The respective mark-position-detecting systems detect respective marks formed on the wafer 74 and on the mask 72. Detection is usually performed optically.
Also, during transfer of a pattern onto the wafer surface, the images of the pattern elements must be formed at the desired locations on the wafer 74. In microlithography in general, the positional accuracy with which pattern elements are formed desirably is less than the minimum line-width of the pattern. The positional accuracy typically is no greater than approximately xc2xc the minimum line-width of the pattern. Consequently, along with improving the resolving power of the projection-exposure apparatus 70 by using X-rays instead of deep-UV light, the positional accuracy with which pattern elements are formed on the wafer 74 by X-ray lithography also must be improved relative to the performance levels achieved with deep-UV lithography. Similar improvements also are needed in overlay accuracy achieved by X-ray microlithography compared to deep-UV microlithography.
To realize improvements in overlay accuracy, corresponding improvements are required in the accuracy with which the respective positions of alignment marks on the mask 72 and wafer 74 are detected. Similar improvements also are required in the accuracy with which the wafer stage 75 is actuated and in the stability of the base line BL (FIG. 6). Base-line stability pertains to the degree to which the base line, from the center of the exposure-image field 62 to the axis of the mark-position-detection system, remains constant in position during a defined time period, e.g., from the time of position measurement to the moment of making an exposure. If the base line becomes unstable, then the exposure position cannot be aligned reliably at the desired location on the wafer, which results in overlay error. Exemplary factors causing base-line variation include deformations of the optical column of the projection-optical system 71 and deformations of structural members supporting the optical column. Consequently, base-line stability may be influenced by lens-column stability, the stability of the mark-position-detection system, the stability of the structural members supporting the optical column and mark-position-detection system, and the length of the base line BL.
With respect to base-line length, a typical sequence of events is: (a) detection of mark position, (b) positional and orientational alignment of the wafer and mask, and (c) actual exposure, in which the wafer stage is moved along the base line from a position at which an alignment mark is detected to an actual exposure position. Hence, a long base line (i.e., a long distance between the mark-detection position and the center of the exposure-image field) results in a correspondingly long distance over which the wafer stage must be driven. Having to move the wafer stage over such a distance degrades the positional accuracy of the stage, with corresponding adverse effects on overlay accuracy.
With conventional X-ray projection-exposure apparatus, it is difficult from a practical standpoint to achieve a sufficiently high base-line stability for obtaining sufficiently accurate overlaying of layers, despite the greater resolving power of X-ray lithography. As a result, conventional X-ray lithography has not produced the desired yield of high-quality micro-electronic devices.
Methods and apparatus as disclosed below address the shortcomings of conventional methods and apparatus as summarized above. One object is to provide X-ray projection-exposure (microlithography) apparatus capable of achieving a desired high overlay accuracy of fine circuit patterns.
According to a first aspect, X-ray projection-exposure apparatus are provided. An embodiment of such an apparatus comprises an X-ray source, an X-ray illumination-optical system, a mask stage for holding the mask, an X-ray projection-optical system, a wafer stage for holding the wafer, and a mark-position-detection system. The source produces an X-ray illumination beam. The illumination-optical system receives the illumination beam from the X-ray source and directs the illumination beam onto a pattern-defining mask, wherein X-ray light reflected from an illuminated patterned area on the mask is a xe2x80x9cpatterned beam.xe2x80x9d The projection-optical system receives and directs the patterned beam so as to form an image on a resist-coated wafer of the illuminated patterned area. The projection-optical system comprises a respective optical column having a center axis, a radius (R) relative to the center axis, and an exposure-image field that is situated off-axis relative to the center axis. The mark-position-detection system detects a position of a mark defined on either the wafer or wafer stage. The mark-position-detection system comprises a respective optical column having a radius (r) and a detection center that is displaced from the center axis of the projection-optical system by a distance greater than (R+r).
The mark-position-detection system can be configured to irradiate, on the mark, detection light that is visible, infrared, or ultraviolet (or a mixture of these) and detects detection light that is reflected, scattered, and/or diffracted from the mark.
The mark-position-detection system can comprise: (a) an optical system having a focal position, and (b) a device for changing the focal position in response to a change in refractive index between the optical system and the mark.
Another embodiment of an X-ray projection-exposure apparatus comprises an X-ray source, an X-ray illumination-optical system, a mask stage, an X-ray projection-optical system, a wafer stage, and a mark-position-detection system as summarized above. In addition, with respect to an x-y coordinate system (defined on the image plane by x- and y-axes having an origin at a point of intersection of the center axis with the image plane), the center of the exposure-image field is situated in a region (x less than 0), and the detection center is situated in a region (xxe2x89xa60) and is displaced from the center axis of the projection-optical system by a distance greater than (R+r). Again, the detection center can be situated either on the x-axis or on a line, parallel to the y-axis, passing through the center of the exposure-image field. In addition, the mark-position-detection system can be configured to irradiate, on the mark, detection light that is visible, infrared, and/or ultraviolet and detects detection light that is reflected, scattered, and/or diffracted from the mark. Furthermore, the mark-position-detection system can comprise: (a) an optical system having a focal position, and (b) a device for changing the focal position in response to a change in refractive index between the optical system and the mark.
According to another aspect, methods are provided for performing X-ray projection-exposure of a pattern, defined by a mask held on a mask stage, onto a wafer held on a wafer stage. In an embodiment of such a method, a beam of X-rays from an X-ray source is irradiated onto a patterned region of the mask to form a patterned beam carrying an image of the irradiated region of the mask. Using an X-ray projection-optical system having a center axis and an image plane, and of which a respective optical column has a radius (R), the image is projected onto an exposure-image field in the image plane on a resist-coated wafer such that the exposure-image field is displaced laterally from the center axis. Using a mark-position-detection system of which a respective optical column has a radius (r), the position of a mark, defined either on the wafer or on the wafer stage, is detected at a detection center of the mark-position-detection system. The detection center is displaced from the center axis by a distance greater than (R+r). In response to a detection signal produced by the mark-position-detection system, the wafer stage is driven so as to form a projection image of the pattern at a desired location on the resist-coated wafer.
In the method summarized above, with respect to an x-y coordinate system defined on the image plane by x- and y-axes having an origin at a point of intersection of the center axis with the image plane, the center of the exposure-image field can be situated in a region (x less than 0) and situating the detection center in a region (xxe2x89xa60).
The method further can comprise the steps of measuring a distance between the origin of the x-y coordinate system and the center of the exposure-image field, and driving the wafer stage a distance determined from the measured distance.
The detection center can be situated either on the x-axis or on a line, parallel to the y-axis, passing through the center of the exposure-image field.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the drawings.